In Windows, the way to see what's still to sync is to look in your file system Explorer. There, there will be your Dropbox folder and those files or folders containing files will have a blue sync icon versus those already uploaded successfully with a green check mark.

Now, it's also possible to see the latest upload events by clicking on the Dropbox icon in the system tray, or to track events online, but afaik, there's no way to see what file will be syncing next.

I'm trying to figure out the same thing. best I've got so far is to look at the sync icons on the folders.

You can at least narrow it down to "which folder contains the files which are currently syncing"...

In my case it's the "Camera Uploads" folder, so I'm going to look on the dropbox website to see what the largest files in that folder are... then check whether they're on my local machine. probably some videos or something.